Pcm telephone signaling



D. C. WELLER PCM TELEPHONE SIGNALING March 26, 1963 3 Sheets-Sheet 1 Filed Oct. 20, 1960 /NVENTOR By D. C. WELLER Q E Q ATTORNEY March 26, 1963 D. c. WELLER PCM TELEPHONE SIGNALING Filed Oct. 20, 1960 3 Sheets-Sheet 2 VVE/V701? BY D. C. WELLER E. @Li- A T TO/PNE V United States Patent O 3,983,267 PCM 'EELEPHNE SIGNALW'G David C. Weiter, Sparta, NJ., assigner to Bell Telephone Laboratories, incorporated, New York, NX., a corporation of N ew Yer-k Filed Get. 2t), 1960, Ser. No. 63,737 it) Ciaims. (Cl. 179-15) -This invention relates generally to pulse type communication systems and more particularly, although in its broader aspects not exclusively, to time division multiplex pulse code modulation systems for use between telephone central oices in the so-called exchange area.

When a number of metallic-pair voice-frequency telephone transmission lines are replaced by a single multichannel carrier trunk, each carrier channel must, if it is to be fully `compatible with the associated switching equipment, be capable not only of carrying the same message information but also of passing the same `form of signaling information as the -voice pair -it replaces. It should, in other words, accept both voice messages and signaling information in the same form that they would have if they were to be impressed upon a metallic pair and should reproduce both in substantially the' same `form at the other end of the line if need for alterations in the associated switching equipment is to be avoided. A number of techniques are known for transmitting signaling information over the respective channels in conventional frequency division multiplex carrier systems, but they are generally not readily appl-icable to time division multiplex systems of the pulse code modulation type. Those that are applicable tend to be so only at the cost of a considerable increase in circuit complexity because of duplication of functions.

Signaling in a telephone system may conveniently be divided into the two broad classications of supervisory and control signaling. The first permits a subscriber or an operator to initiate a request for service, holds or releases a connection after it has been established, or recalls an operator on a previously established connection. The second permits information to be passed over the line to direct the establishment of a particular desired connection. Since, at least -between central otiices in the exchange area, the type of signaling generally encountered that places the most stringent requirements on the system is a form of control signaling known as revertive pulsing, it is to revertive pulsing that the description of the present invention is primarily directed. The principles underlying the invention are, of course, applicable to other forms or" signaling as well, particularly to others based upon the rapid transmission of pulse information in either one or both directions over the trunk.

Revertive pulsing between telephone central oiices owes its existence to the nature of the operation of certain types of central oliice switching equipment. Some switching devices, notably panel selectors, are driven by their own power over banks of terminals and signal their position by producing pulses as the respective terminals are passed over. In each such installation, these pulses are counted and the switch is stopped when the desired position is reached. The oper-ation of such equipment from a central oiiice at the remote end of a trunk is known as revertive pulsing since that oflice controls the setting of the switches by means of pulses which revert back over the trunk. The central oihce at which the call originates generates only start and stop indications in the form of busy and idle signals.

ln general, revertive pulsing trunks originate calls only in one direction. The central oice at which the call is initiated is termed the originating oiiice, while that at which the call is received is known as the terminating ofce. The terminating otIi-ce normally supplies the originating office with a negative voltage known as battery on the so-called tip side of the line Iand ground on the so-called ring side to indicate an on-hook condition at the terminating oiiice. The originating office transmits a start signal by becoming busy, causing current to flow in the terminating office. The terminating oflice then sends .back momentary short circuits or ground pulses as its switching equipment passes over its terminal contacts. When the proper number of `ground pulses have been received, the originating oice transmits a stop signal by becoming idle and interrupting the flow of current. The switching equipment at the terminating olice then advances to the next selector and the sequence is repeated as often -as necessary until the called number is reached. When the called subscribed answers his telephone, the terminating otiice reverses the polarity of the direct voltage supplied to the originating oce, placing ground on the tip side of the yline and battery on the ring side.

In any multichannel carrier multiplex system, itis clearly not possible to transmit such two-state direct-current signals directly over the facility Ibetween central otiices without modification. Direct currents and voltages cannot conveniently be transmitted and it is important that each channel be able to signal independently of and 'without disturbance to the others. In conventional frequency division multiplex systems, it is common to transmit signaling information by either pulsing the amplitude or shifting the frequency of one or more tones which are either Within or just outside of the voice-frequency band and using them to modulate the carrier in the same manner as do the regularly transmitted speech waves. Application of such techniques to a time division multiplex pulse code modulation system, however, would tend to require a large'amount of equipment in each channel for the generation land control `of the tones prior to coding.

A principal object of the present invention, therefore, is to transmit signaling information over a multichannel pulse code modulation telephone system with a maximum of speed and with a minimum of circuit complexity.

A closely related object is to transmit signaling information over a multichannel pulse code modulation telephone system without degrading voice-frequency message transmission during the commercial portions of telephone calls.

In accordance with one important feature of the invention, two-state direct-current signaling information is transmitted over a multichannel pulse code modulation message transmission system employing a predetermined number of digit spaces per channel for message transmission by adding a further digit space to each channel exclusive of the message digit spaces and transmitting binary l in the added digit space in response to one state of the direct-current signal and binary 0 in response to the other state. The original direct-current signaling state is reconstructed at the opposite end of the ysystem in the corresponding channel in response to the content of the added digit space. Signaling information is, in this manner, transmitted rapidly over the pulse code modulation system with a minimum of additional circuitry and with no interference between channels. Since the added digit space is not one of those required for message transmission, there is, moreover, no accompanying degradation in the quality of message transmission. The principal cost is a relatively small increase in system bandwidth requirements.

While a single signaling path per channel is adequate to meet most telephone signaling requirements, it is still not of itself suicient to permit revertive pulsing. While a revertive pulsing trunk needs only one signaling pathin the outward direction, to transmit idle or busy signals, it requires two signaling paths in the opposite or inward absage? n D direction, one to transmit revertive pulses and the other to transmit on-hook or off-hook signals. This invention, in addition to providing a single high-,speed signaling path in each direction in the manner described above, also furnishes a second signaling path which is available for use in the inward direction to meettherequirements for revertive pulsing.

According to a second important feature of the invention, two-state direct-current signaling information is transmitted overy a multichannel pulse: code vmodulation system employing'a predetermined number of digit spaces per channel for message transmission not only by adding a further digit space to eachchannel exclusive lof the message digit lspaces and transmitting binary kl.in the added digit space in response to one state of a iirst direct-current signal and binary in response to the other but also by transmittingbinary -l in the least signiiicant message digit space of the channel concerned inresponse toone state and binary 07 in response to the other of a second direct-current signal. 11n a revertive pulsing ytelephone trunk, according to this feature ofthe invention, one of the signalingpaths sov provided is used to transmit onhook or off-hook signals', while the otheris used to transmit revertive pulses. Both paths provided rapid signal transmissionwith a minimum of circuit complexity and the slight increase offnoise which takes place within the voice-frequency range with the addition of the second signaling path is made to do so onlyduring the noncommercial portions ofa telephoney call.

.ln at least one embodiment of the invention non-interference with voice-frequencymessages in the/,direction back toward the originatingolliceis assured A, by transmitting binary fl in the. added digit space to designate `the presence of battery, transmitting binary ,0fin ,that digit space todesignateA afshort circuit or revertive pulse, Vforcing the transmission of binary 0 during the least signincant digit space to designate normal battery polarity, and permitting binary rli to lie-transmitted at least randomly in the least significant messagedigit to designate battery reversal. Sincebattery reversal is transmitted from the terminating cnice as an off-,hook signal, the least significant digit space is thus available for messagepse during the commercial-portionofa ca ll. l.'Ihere is some slight performance degradation during van en -hook conditionrdue to in-band noise caused byloss ofthe least s igniiicant digit space, butthe channel is used for messagetransmission at such times only by trained operators who will experience uordifsultrbesaess Qfit.V Y n Y. a

t Other objects andfeatures of the i vention will become apparent from the following description of one specilc embodiment arranged to prov-ide reveriive pulsing between telephone .centralioices inra-multichannel time division multiplex pulse code modulation system, Infthe drawings:

IFIG' l shows the pulse code modulation equipment used at the originating central oflice in an embodiment of vthe invention; t ,Y Y. e

FIG. 2 illustrates thefpulse code modulation equipment used .attire terminating central ofticein the same embodiment ofthe invention; i Y.)

-FIG.3 illustrates several of the control pulse waveforms used in the embodiment of the invention shown in FIGS., Land 2; and

1'FIGS.;4fand 5 are charts showing the operation ofthe circuitryofvFIGS. l and Zunder thev different signaling conditions required for revertive pulsing.

,T 'he'vrnultichannel timedivision multiplex pulse ,code modulation telephoneltrunlrv illustrated in FIGS. l and 2 is used to replace a multiplicity of metallic-pair voicefrequency linesrbetweenl `telephone central offices. Only one channel is illustrated in `detail ateach oilice, but the pointsat which thev remaining channels are connected are indicated. As shown, this channel terminates yin a two-wire voice-frequency line at each central oce where it; is connected to the usual central ofiice switching equipment. As indicated above, theY trunk is equipped for revertive pulsing to illustrate the application of the principles underlying the present invention.

In FIG. l, the two-wire voice-frequency line 1l from the originating centnal office is connected to a hybrid network 12 which is, in turn,` terminated by a suitable balancing network 13. 'Irans-mission beyond hybrid net- Work 12 is on a four-wire basis, with the two directions of transmission separated by the conjugacy of hybrid network 12. 'I'he transmitting path from hybrid -network 12 takes the form of a low-passilter 14, a channel sampling gate 15, 4a compressor 16, -a pulse code modulation encoder y17, and a transmitting regenerative pulse amplifier 18.V Filter 14 serves to limit the -top frequencies of the transmitted voice-frequency messages to 4 kilocycles, for

example, and gate 15 is enabled -by 1a so-calied channel pulse at an 8Akilocycle rate. As shown in the rst four lines of FIG. 3, the channel pulses assigned to each channel are displaced in time from all other channel pulses, with the result that the input applied to compressor 16 is a time-division multiplexed sequence of samples from all of the channels being transmitted. As indicated, samples from theotherv channels are interleaved with those from gatevlS at the input of compressor 16.

AsV is now common practice in many multichannel carrier systems, the embodiment of the invention illustrated in FIGS. l and 2 is provided with oompandcrs in order to improve the signal-to-noise ratio of the system. In each direction of transmission, a compander takes the form of an instantaneous volume compressor Lat the transmitting terminal followed by `a complementary instantaneous volume expander at the receiving terminal. In a pulse code n modulation system, the elect of the compander is to increase thepercentage of the encoder volume range that is used by low volume mess-ages, thereby reducing the amount of so-called quantizing noise. High volume messages already take full advantage of the encoder range, so they are already transmitted with a minimum amount of quantizing noise.

,Compressor 16 in the transmitting path in FIG. l is followed lby pulse code modulation encoder 17. As shown, encoder 17 is a seven-digit encoder. It employs seven message digits per channel in vthe time scale, in other words, to translate each compressed sample applied to it` into a binary code group of marks (binary l) and spaces (binary 0)A occupying seven consecutive digit spaces o r time slots. To control the timing of encoder 17,V timing pulses which recur during the same numbered time slot of each code group are applied to the timing control leads, Dl through D7. As shown in the iifth line of FIG. 3, the Dl lead is energized during the rst time slot or digit space of each code group and controls the timing of the generation of the marks or spaces in the most significant digit space. As shown 'in the sixth line of FIG. 3, the D7 lead-is energized during the seventh time slot of each code group and controls the timing of the generation of the marks or spaces in the least signiiicant digit space. The D2 through D6 leads are energized in a similar manner during their respective digit spaces or time slots. Encoder 17 may, by way of example, take the form of the network type of encoder disclosed in application VSerial No. 744,190, which was led June 24, 1958 by R. E. Yaeger.

The transmitting regenerative pulse amplier 18 is connected to receive the multiplexed binary code groups generated by encoder 17 and serves to insure uniformity of pulses for transmission over the trunk to the receiving equipment at the terminating central office.

The portion of the originating folice transmitting circuitry which has thus far been described is convention-al at the present stage of development of the pulse code modulation art. The remaining transmitting circuitry illustrated in FIG. l is, in 4accordance with the invention, provided to furnish a high-speed signaling path from the origina-ting oice to the terminating .oilice and to com-v plete the two signaling paths directed from the terminating oce back toward the originating oiiice.

Near the left-'hand end of two-wire voice line .11 in FlG. 1, normally closed relay contacts 21 and 22 are connected in series in the tip and ring sides (marked T and R, respectively) of the line. Cross-connections between the tip and ring sides of the line contain normally open relay contacts 23 and 24. These relay contacts, as will be explained later, belong to a so-called SUP (-for supervisory) relay controlled Ifrom the terminating oice and are used to reverse battery polarity on line y11 in response to an oli-hook signal. Farther to the right, resistors 25 and 26 are connected in series with the tip and ring sides o'r the line, respectively. Still farther to the right, relay contacts 27, which are open when their relay is operated, ground the tip side of the line and relay contacts 28, which are closed when the same relay is operated, conneet the tip side of the line to hybrid l12 and through a resistor 29 to a negative 48-volt source 30. Relay contacts 27 and 28 belong to a so-called RP (for revertive pulse) relay controlled from the terminating oice and are used to connect a substantially short circuit across the line to reconstruct revertive pulses. To the right of resistor 26, the ring side of the line is connected to hybrid 12 and is grounded through a small resistor 3-1. Resistors 25, 26, 29, and 31 form an impedance matching pad While relay contacts 27 are open and relay contacts 28 are closed. As will be explained later, this is the normal condition of the RP relay. During a revertive pulse, however, contacts 27 are closed, contacts 28 are open, .and the resistors simulate the impedance of a direct two-wire metallic trunk.

The high-speed signaling path to the terminating otlce aiorded by the invention begins just to the left of resistors 25 and 26, where a pair of isolating resistors 34 and 35 are connected to the ring and tip sides of the line, respectively. A pair of capacitors 36 and 37 are connected to ground from the other ends of isolating resistors 34 and 35 to prevent any stray alternating-current components from adversely affecting the performance of the signaling path. The end of isolating resistor 34 is also connected through the series combination of a resistor 38, a blocking capacitor 39, and a resistor 40 to a regenerative pulse amplier 41, while the end of isolating resistor 35 is also connected through the series combination of a blocking capacitor 45, a resistor 46, and a diode 47 to the junction between resistor 38 and capacitor 39. Diode 47 is poled for easy current llow toward the junction between resistor 38 and capacitor 39 and is a switch which operates to control a so-called SIG (for signaling) relay at the terminating oice. The junction between resistor 46 and diode 47 is connected lthrough a blocking capacitor 48 to the same source of channel pulses that drives sampling gate 15. To complete the circuit, the junction of resistors 34 and 318 is returned to ground through a resistor 49, and that of capacitor 45 and resistor 46 is returned through a resistor 50 to a negati-ve 34-Volt source 51. Similar signaling path circuitry in other channels is connected into the common transmitting path at the input side of regenerative pulse amplifier 41, and the output of amplilier 41 is connected through an AND ygate 54 to the input side of transmitting amplifier 18. An AND gate, as is well known, is enabled only when all of its input leads are energized. AND gate 54 is, in acordance with an important feature of the invention, supplied at its other input lead with digit pulses during an eighth digit space assigned to each channel. This eighth digit space is, as has already been pointed out, exclusive of the digit spaces used in each channel for message transmission and its use for the transmission of direct-current signaling information does not, therefore, detract from the quality of voice-frequency message transmission. The only cost of the additional digit space is approximately fteen per- 6 cent in additional bandwidth. The eighth-digit control pulses are shown in the seventh line of FIG. 3.

The signaling circuitry which has just been described is the SIG detector and monitors two-wire line 11 to determine whether or not current is flowing. When the originating office is idle, the office end of line 11 is open circuited and no current flows. Under such conditions, diode 47 is back biased by negative 34-volt source 51 and transmission of channel pulses to the input of pulse amplifier 41 is blocked. When the originating office is busy, on the other hand, the oce end of line 11 is short circuited and negative f8-volt source 36 forward biases diode 47, permitting channel pulses to reach amplier 41. As will be explained later, the relay controlling contacts 27 and 28 is operated except when a revertive pulse is being transmitted and, except during such intervals, negative 48-volt source 30 is connected to the line. With the addition of AND gate 54, binary 1 is thus transmitted during the eighth digit space in any channel in response to a busy or closed loop condition and binary 0 is transmitted during the eighth digit space in response to an idle or open loop condition. This operation is tabulated for easy reference in FIG. 4.

The message and signaling receiving circuitry at the other end of the carrier line is illustrated in FIG. 2. There, a receiving regenerative pulse amplifier `60 recovers and regenerates the pulse pattern transmitted by amplier 18 in FIG. 1. The -output of receiving arnplier 60 is passed through a pulse code modulation decoder '61 which may, `for example, be `the type disclosed in United States Patent 2,991,422, issued July 4, 1961, to R. E. Yaeger. Control pulses are supplied to decoder 6|1 during digit spaces D1 through D7 in each channel to control decoder timing. The output of decoder `61 is passed through an expander 62 and the output of the latter is in the -form of the same succession of message samples as applied to compressor 16 in FIG. 1. Message distribution to each of the receiving channels takes place at that point.

In the illustrated receiving channel, the output side of expander 62 is connected through a channel gate 63 and a low-pass filter 64 to a hybrid netwonk 65'. The channel pulses of the channel concerned are applied to channel gate `63 to separate the message samples of this channel.

Ifrom those of others. The channel pulses for the iirst -four channels are, it will be recalled, illustrated in the rst four lines of FIG. 3. Low-pass -lter 64 removes the high-frequency components from the message samples for this particular channel and restores the message to its original form. Hybrid `65 is terminated by a balancing network 66.

To the right of hybrid netwonk 65, transmission is once again on a two-wire basis through a transmission line I67. A -small resistor 68 is connected bet-Ween the ltip and ring sides of the two-wire yline 67 just to the right of hybrid network 65' to provide direct-current contlnuity, and a pair of small resistors 69 and 70V are connected in series with the tip and ring sides of line `67, respectively, to form, with resistor 68, an impedance matching pad and to simulate the impedance of a metallic palr.

The portion of the terminating otiice receiving circuitry which has been described is largely conventional. The remaining receiving circuitry is, in accordance with the invention, provided to complete the high-speed signaling path from the originating otice.

Between resistors 68 and 70, a pair of normally open relay contacts 71 are connected in series with the ring side of line 67. These contacts belong to a so-called SIG (for signaling) relay and are used to create open and closed loop conditions on line 67 in response to idle or busy signals received from the originating ofce. Relay contacts 71 are bypassed by a capacitor '72 to pro- 7 vide a voice-frequency transmission path when=they are in their open condition.

The exclusively signaling circuitry in FIG. 2 begins at the right of receiving amplifier 6i), where one input lead of an AND gate 75 is connected to the line. The other input lead 4of AND gate 75 is, in accordance with a feature of the invention, supplied with digit pulses during the eighth digit space assignedV to each channel. For the channel illustrated, the output lead of `AND gate 75 is connected :through another AND gate 76 (which is controlled bychannel pulses) and an integrating circuit 77to -a SIG (for signaling) relay 7S. This circuitry is the SIG regenerator and, as shown in FIG. 4, serves to operate relay 73 in response Ato binary 1 received during the eighth digit space and yto release itin response to ybinary received during the eighth digit space. The func-tion of integrating circuit 77 is to build up sufiicient current to provide rapid operation of relay '.78 and to hold its state of operation between successive driving digit pulses.

Relay contacts 71 in FIG. 2 are the contacts of SIG relay 78. As indicated inFIG. 4, they are open in the presence of binary 0 in thereighth `digit space, thus recreating an open loop condition on line 67 as an idle signal. They are closed in the presence ofd binary 1 in the eighth digit space,k recreating a closed loop condition as a busy signal. The direct-current signaling state recreated on line 67 is, therefore, substantially identical to the one originally appearing on two-wire line 11 in FIG. l, permitting switching equipment at both central ofiices :to operate just as ifno pulse code modulation system intervened. This is accompanied,.moreover, with a maximum of speed and precision and with a minimum of added circuit complexity. l

The pulsecode modulation Vequipment in the direction from the terminating oice to the originating oice is substantially the same las that inthe opposite direction which has already been described. A low-pass `filter 85 is connected to hybrid network 65 and is isolated from filter 64 in the receiving path by the'conjugacy of hybrid network 65. Beyond filter 85 in the transmitting path are a channel gate 86, a compressorV 87, and la Vpulse code modulation encoder 8S. Other channels are multiplexed for transmission at the input side of compressor 87. The output of encoder 8S is, for reasons which will be explained later, connected .to a transmitting regenerative pulse amplifier 89 through `an `INI-IIBIT gate 90. Such a gate is disabled whenever its INHIBIT terminal (indicated lby the smaller semi-circle in the symbol) is energized.

As has already been explained, the present invention provides two high-speed signaling paths from the terminating oflice back to .the originating office. VAs shown in tabular form in FIG. 5, these paths Iare used to transmit `on-hook or off-hook signals and to transmit revertive pulses.

At the terminating ofiice, as shown in HG. 2, the signaling states supplied by the central office switching equipment to two-wire line `67 are monitored by a pair of detectors. One of these, the SUP (for supervisory) detector, detects battery polarity while the other, the RP (for revertive pulse) detector, detects ground pulses.

Near the right-hand end of two-wire line l67 in FIG. 2, a pair of isolating resistors 94 and 95 are bridged across the line, with resistor 94 connected to the tip side and resistor 95 connected to the ring. A pair of capacitors 96 and 97 are connected -to ground from the other ends of resistors 94 and 95 to bypass stray alternating-current components and keep them from Vaffecting the performyance of .the two detectors. The end of resistor 95 is also connected through the series combination of a resistor 98, a blocking capacitor 99, and a resistor 10%) to a regenerative pulse amplifier 101, while the end of resistor 94 is also connected through a resistor 102V and a diode 163 to the junction between resistor 9S and capacitor 99. .Diode 103 'is poled for easy current flow toward the junction between resistor 98 and capacitor 99 and serves as a switch, the `action of which ultimately controls a so-called supervisory relay at the originating cfice. The junction between resistor 162 and diode iti-3 is connected through a blocking capacitor 19e to the same source of channel pulses that drives sampling gate 8'6. The circuit is completed by a resistor 1197 returned to a positive S-volt source 163 from the junction between resistor 98 and capacitor 99 and by `a resistor 109 returned to a negative S-volt source 118 from `a junction between resistorli?. and'diode 193.

Another pair of isolating resistors 114 and 115 are also connected to the ends of resistors 94 and 95, respectively. VThe other ends of resistors j114 and 115, however, are joined together and connected through a blocking capacitor 116 and a resistor 117 to a regenerative pulse lamplifier V118. rthe junction between resistors 114 and v115 is also connected through a diode 119 and a resistory 120,10 a negative 8-volt source 121. Diode 119 is. poled for easy current flow in the direction 4toward resistors 114 and 115 andthe junction between diode 119 and resistor 12) is supplied with channel pulses through a blocking capacitor 122.

ri`hel detector circuits shown in FIG. 2 are completed byJan INHIBIT gate y125 and an AND gate 126. The former has its INHiBlT terminal supplied from the output of regenerative pulse amplifier 101 and its remaining input terminal supplied with pulses during the seventh digit space of each channel. The output of IN- HBIT gate 125 is supplied to the INHIBIT terminal of INHIBIT gate 90. AND gate 126, on the other hand, has one input terminal supplied by the output of regenerative pulse amplifier 11S and the other supplied withpulses during the eighth digit space of each channel. The output of AND gate 126 is supplied to the input side of transmitting regenerative pulse amplifier 89.

The signaling circuitry which has just been described makes up the SUP (for supervisory) and RP (for revertive pulse) detectors. These detectors monitor two-wire line 67 both Vto determine whether or not current is flowing and. to determine the direction of current flow. When the central office switching equipment supplies a revertive pulse to line 67, the tip and ring sides of the line are shorted toV one another and no current iiows. Current fiows in line 67 at all other times, the direction of flow depending upon whether an on-hook or oit-hook condition is present.

In the absence of a revertive pulse, the central oftice switching equipment applies a direct voltage, termed battery, to two-wire voice line 67 in FIG. 2. For an onhook condition, so-called normal battery is applied. In this condition, the tip side of the line is held at a negative iS-volt potential while the ring side is grounded. The SUP diode 133 is back biased, blocking channelpulses from the INI-EBIT lead of gate 125. Gate 125 passes digit pulses during the seventh or least signiricant message digit space, under such conditions, to the INHIBIT lead of gate 90. Gate blocks transmission from encoder 3S by forcing the transmission of binary O (or in other words, by forcing the transmission of a space) during the seventh digit space, as `shown in tabular form in FIG. 5. An on-hook condition is, in accordance with an important feature of the invention, thus indicated by the forced transmission of binary 0 during the seventh or least significant message digit space. Since six digit spaces are still available for message transmission, an operator may, if necessary, talk in an ori-hook condition with but a slight loss in transmission quality. Such loss in transmission quality can never take place during the commercial portion of a call. The second signaling path provided by the invention does not, therefore, have any adverse effect upon the transmission quality of the system as used by subscribers.

Because isolating resistors 114. and are connected 9 together, their junction with RP diode 119 is held at a negative ilil-volt potential under normal battery conditions and diode 119 is forward biased, permitting channel pulses to pass to AND gate 126. AND gate `126 thus passes pulses to transmitting amplifier 89 during the eighth or added digit space, as shown in FIG. 5, indicating the absence of a revertive pulse.

For an off-hook condition, the terminating central oiiice equipment reverses the battery polarity applied to twowire line 67. The ring side of the line is held at a negative iS-volt potential, while the tip side is grounded. The SUP diode 103 is then forward biased and channel pulses can pass through amplifier 101 to the lNHlBIT lead of gate 12S. The seventh or least signiiican-t message digits are inhibited by gate 125. Gate 90 is, therefore, not inhibited during the seventh digit space by pulses at its INHlBlT lead and any pulses generated in the seventh digit space by encoder 8S pass freely to transmitting regenerative pulse amplifier 89. When a channel is not in use, there is normally enough random noise on the line to cause encoder 8S to generate binary l randomly in the seventh digit space. When a channel is in use, as it normally is in the off-hook condition, such noise plus the transmitted message ensures at least the random presence of binary l in the seventh or least signilicant message digit space. As tabulated in FIG. 5, therefore, an offhook condition is indicated by the random presence of binary l in the seventh or least significant message digit space of the channel concerned. The invention thus permits full use of all seven message digit spaces in the offhook condition or, in other words, during the commercial portion of a call. Binary "1 is transmitted during the eighth or added digit space, as before, indicating the absence of a revertive pulse.

'In general, revertive pulsing takes place only while the terminating ofice is in an on-hook condition or, in other words, while the ring side of two-wire line 67 is grounded and the tip side is connected to negative AFS-volt battery. In the absence of a revertive pulse, the junction between resistors 114 and 115 is, as has already been pointed out, at approximately a negative 2li-volt potential. 'Ihe RP diode 119 is forward biased and channel pulses are passed to regenerative pulse amplifier 118. When the central oiiice switching equipment applies a revertive pulse to two-wire line 67, the tip side of the line is grounded and the `junction between resistors 114 and `115 is at substantially ground potential. This reverse biases RP diode 119 and blocks channel pulses from regenerative pulse amplifier 11S. Binary 0, rather than binary 1, is therefore transmitted during the eighth or added digit space of the channel concerned. The symmetrical connection of RP diede 119 to the tip and ring sides of line 67 makes operation of diode 119 independent of longitudinal line noise.

As shown in FIG. l, the pulse code modulation message transmission path is completed at the originating oiice by a receiving regenerative pulse amplifier 130 connected to hybrid network 12 through the series combination of a pulse code modulation decoder 131, an expander 132, a channel gate 133, and a low-pass filter 134. These `components are substantially identical to the corresponding components at the terminating oice shown in FIG. 2 and will not be redescribed.

To complete the two signaling paths provided by the invention, the originating otiice equipment shown in FIG. l employs two signaling regenerators, both connected to the incoming line at the output side of receiving amplifier 13G. Both regenerators closely resemble the SIG regenerator shown in FIG. 2. The first of these, -termed the SUP (for supervisory) regenerator, is a tandem chain made up of a pair of AND gates 138 and 139, an integrating circuit 146, and a slow-release SUP relay 141. As indicated in FIG. l, SUP relay 141 controls polarityreversing contacts 21 through 24 in two-wire line 11. AND gate 138 is driven by digit pulses during the seventh or least significant message digit of each channel, while AND gate 139 is driven by the channel pulses of the channel concerned. Corresponding channel gates to other channels are also connected to the output side of AND gate 138. integrating circuit 1140 functions similarly to integrating circuit 77 in FIG. 2 but has a longer time constant since binary 1 is only randomly present in the seventh message digit space during transmission of an oil-hook signal from the terminating central ofce. SUP relay 141 is a slow-release relay for the same reason.

The final originating office signaling regenerator is the RP for revertive pulse) regenerator. 1t is made up of an AND gate 145 connected in series with an AND gate 146, an integrating circuit 147, and a RP relay 148. rthis regenerator is like the SUP regenerator except that AND gate 145 is driven by digit pulses during the eighth or added digit space and integrating circuit lla-'7 has the shorter time constant .of integrating circuit 77 in FiG. 2. As indicated in FiG. l, RP relay 148 controls relay contacts 2.7 and 2S in two-wire line 11.

The SUP and RP regenerators operate to reproduce on two-wire line 11 at the originating oiiice Isubstantially the identical direct-current signaling information supplied to two-wire line 67 at the terminating oilice by the local central oiiiee switching equipment. The manner in which this is done is tabulated in FIG. 5. Thus, when two-wire line 67 is on-hook at the terminating oiiiee, the originating Office equipment illustrated in FIG. 1 receives binary Oi in the seventh or least signiiicant message digit space and binary 1 in the eighth or added digit space. The binary 0 in the -seventh digit space `leaves relay contacts 21 and 22 closed and relay contacts 23 and 24 open, while the binary l in the eighth digit space opens contacts 27 and closes` contacts 28. The condition thereby reconstructed on two-wire line 11 is substantially identical to that detected at the terminating office, namely, a negative 48-volt potential on the tip side of the line and ground on the ring.

Binary il is received during the eighth digit space when a revertive pulse is detected .at the terminating ofi-ice. Such signals close RP relay contacts 27 and open RP relay contacts 28, providing a substantial short circuit between the ring and tip sides of two-wire line 11.

Finally, when two-wire line 67 goes off-hook at the terminating otiice, the originating oice equipment receives -binary 1, at least randomly, during the seventh digit space of the channel concerned and binary l during the eighth digit space. Operation of SUP relay 141 by random seventh digit pulses opens contacts 21 and 22 and closes contacts 23 and 2li, effectively reversing the polarity of the direct voltage on two-wire line 11. The ring side of the line is thus connected to a negative 4S-volt potential and the tip side is grounded. As before, the pulse during the eighth digit space opens RP relay contacts 27 and closes RP relay contacts 28.

1t is to be understood that the above-described arrangement is illustrative of the application of the principles of the invention. Numerous other arrangements may be devised by those skilled in the art without departing from the spirit and scope of the invention.

What is claimed is:

1. In a pulse code modulation message transmission sy-stem interconnecting a pair of terminals and employing regularly recurring code groups each containing a predetermined number of successive digit spaces of substantially equal time duration for message transmission, an arrangement for transmitting a two-state direct-current signal between said terminals which comprises means for adding a further digit space exclusive of and in sequence with said message digit spaces to each of said code groups, said further digit space having substantially the same time duration as each of said message digit spaces, means for detecting the state of the direct-current signal at one of said terminals, means to transmit one binary state in said added digit space in response to one state of said direct-current siffnal and the other binary state in response to the other, and means to reconstruct the detected state of said directmission, an arrangement for transmitting a pair of twostate direct-current signals simultaneously in the same direction between said terminals which comprises means for adding a further digit space exclusive of said ymessage digit spaces, means for detecting the state of a iirst of said direct-current signals at a first of said terminals, means to transmit binary l in said added digit space in response toone state of said rst direct-current signal and binary O in response to the other, means for detecting the state of the second of said direct-current signals at said first terminal, means to transmit binary l in the least significant of said message digit spaces in res"onse to one state of said second direct-current signal and binary D in response to the other, means to reconstruct the detected state of `said first direct-current signal in response to the content of said added digit space at the second of said terminals, and means to reconstruct the detected state of said second direct-current signal in response to the content of the least signicant of said message digit spaces at said second terminal.

3. In a pulse code modulation message transmission system interconnecting a pair of terminals and employing a predetermined number of digit spaces for message transmission, an arrangement for transmitting a pair of twostate direct-current signals simultaneously in the same direction between said terminals which comprises means for adding a further digit space exclusive of said message digit spaces, means for detecting the state of a rst of said direct-current signals at'a rst of said terminals, means to transmit a pulse in said added digit space only in response to one state of said rst direct-current signal, means for detecting the state of the second of said direct-current signals at said first tenminal, means 'to force transmission of binary O in the least signicant of said message Vdigit spaces in response to one state of said second direct-current signal, means to permit the presence of binary l in the least significant of said message digit spaces in response to the other state of said second direct-current signal, means to reconstruct the detected state of -said first directcurrent signal in response to the content of said added digit space at the second of said terminals, and means to reconstruct the detected state of said second direct-current signal in response to the content of the least significant of said message digit spaces at said second terminal.

4. In a multichannel pulse code modulation message system interconnecting a pair of terminals and employing a predetermined number of digit spaces per channel for message transmission, an arrangement for transmitting direct-current signaling information over each channel between said terminals which comprises means for adding a further digit space to each channel exclusive of the message digit spaces, means in each channel for detecting the signaling state -at one of said terminals, means to transmit one binary state in the added digit space in each channel in response to one signaling state and the other binary state in response to the other, and means Vto reconstruct the detected signaling st-ate in each channel at the other of said terminals in response to the content of the corresponding added digit space.

5. In a multichannel pulse code modulation message system interconnecting a pair of terminals and employing a predetermined number of digit spacesrper channel for message transmission, an arrangement for transmitting a pair of two-state direct-current signalssimultaneously Vin the same direction between said terminals which comprises means for adding a further digit space to each channel exclusive of the message digit spaces, means in each channel for detecting the state of a first of said direct-current signals at a first ofsaid terminals, means to transmit one binary state in said added digit space in each channel in response to .one state of said first directcurr-ent signal and the other binary state in response to the other, .means in each channel vfor detecting the state of the second of said direct-current signals, means to transmit one-binary state in the least significant of said message digit spaces in each channel in response to one state of said second direct-current signal and the other binary state in response to the other, means to reconstruct the detected state of said lirst direct-current signal in each channel at the second of said terminals in response to the content of said added digit space, and means to reconstruct the detected state of said second direct-current signal in each channel at'said second terminal in response to the content of the least significant of said message digit spaces.

6. In a multichannel pulse code modulation message system interconnecting a pair of terminals and employing a predetermined number of digit spaces per channel for message transmission, an arrangement for transmitting a pair of two-state direct-current signals simultaneously in the same direction between said terminals which comprises means for adding a further digit space to each -channel exclusive of the message digit spaces, means in each channel for detecting the state of a lfirst of said direct-current signals at a first of said terminals, means to transmit binary l in said yadded digit space in each channel only in response to one state of said first directcurrent signal, means in each channel for detecting the state of the second of said direct-current signals, means to force transmission of binary O in the least signicant of said message digit spaces in each channel in response to one state of said second direct-current signals, means to permit the presence of binary l in the least signilicant of said message digit spaces in each channel in response to the other state of said second direct-current signal, means to reconstruct the detected state of Said iirst direct-current signal in each channel at the second of said terminals in response to the content of said added digit space, and means to reconstruct the detected state of said second direct-current signal in each channel at said second terminal in response to the content of the least significant of said message digit spaces.

7. In a pulse code modulation telephone system interconnecting a pair of terminals Vand employing a predetermined number of digit spaces for voice-frequency message transmission, terminating two-wire telephone lines vfor said system at each of said terminals and an arrangement for transmitting the idle or busy condition of the terminating line at a irst of said terminals to the second of said terminals which comprises means for adding a further digit Aspace exclusive of said message digit spaces, means for detecting the idle or busy condition of the terminating line at said lirst terminal, means to transmit binary l in Said added digit space in response to a detected busy condition, means to transmit binary O m said added digit space in response to a detected idle condition, means for imposing a closed loop condition on the terminating line at said second terminal in response to a received binary l in said added digit space, and means for imposing an open loop condition on the terminating line at said second terminal in response to a received binary 0 in said added digit space.

8. In a pulse code modulation telephone system interconnecting -a pair of terminals and employing a predetermined number of digit spaces for voice-frequency message transmission, terminating tWo-wire telephone lines for said system at each of said terminals, an arrangement for transmitting the oil-hook or off-hook condition of the terminating line at a first of, said terminals to the second of said terminals which comprises means for detecting the on-hook or olf-hook condition of the terminating line at said first terminal, means to force transmission of binary 0 in the least significant of said message digit spaces in response to a detected ori-hook condition, means to' permit the presence of binary l in the least signicant of said message digit spaces in response to a detected ofi-hook condition, means for imposing a direct voltage of one polarity on the terminating line at said second terminal in response to received binary O in said least signicant digit space, and means for imposing a direct voltage of the opposite polarity on the terminating line at said second terminal in response to received binary l in said least significant digit space and an arrangement for transmitting revertive pulses from the terminating line at said rst terminal to said second terminal which comprises means for adding a further digit space exclusive of said message digit spaces, means for detecting revertive pulses on the terminating line at said lirst terminal, means to transmit Ibinary l in said 4added digit space in the absence of a detected revertive pulse, means to transmit binary in said added digit space in response to a detected revertive lpulse, and means for substantially shorting the direct voltage on the terminating line at said second terminal in response to a received binary 0 in said added digit space.

9. In a multichannel pulse code modulation itelephone system interconnecting a pair of central ofiices and employing a predetermined number of digit spaces per channel for voice-frequency message transmission, separate terminating two-Wire telephone lines for each of said channels at each of said central oices, an arrangement for transmitting the idle or busy condition of each of said terminating lines at a first of said central offices to the corresponding terminating line at the second of said central oiiices which comprises means for adding a further digit space to each channel exclusive of said message digit spaces, means in each channel `for detecting the idle or busy condition of the corresponding terminating line at said lirst central oiiice, means in each channel to transmit binary l in said added digit space in response t0 a detected Ibusy condition, means in each channel to transmit binary 0in said added digit space in response t0 a detected idle condition, means in each channel for imposing a closed loop condition on the corresponding terminating line at said second central oi'lice in response to a received binary l in said added digit space, and means in each channel -for imposing an open loop condition on the corresponding terminating line at said second central of`n`ce in response to a received binary 0 in said added digit sp-ace.

10.1ln a multichannel pulse code modulation telephone system interconnecting a pair of central ofces and employing a predetermined number of digit spaces per channel for voice-frequency message transmission, separate terminating two-wire telephone lines for each of said channels at each of said central oliices, an arrangement for transmitting the on-hook or oli-hook -condition of each of said terminating lines at a rst of said central ofiices to the corresponding terminating line at the `second of said central oliices which comprises means in each channel for detecting the on-hook or oli-hook condition of the corresponding terminating line at said iirst central oiice, means in each channel to force transmission of binary 0 in the least significant of said message digit spaces in response to a detected on-hook condition, means in each channel to permit the presence of binary 1 in the `least signicant of said message `digit lspaces in response to a detected oidhook condition, means in each channel for imposing a direct voltage of one polarity on the corresponding terminating line at said second central oliice in response to received binary 0 in said least significant message digit space and means in each `channel for imposing a ldirect voltage of the opposite polarity on the corresponding terminating line at lsaid second central oiiice in response to received binary l in said least significant message digit space andan arrangement for transmitting revertive pulses from each of said terminating lines at said rst central oliice to the corresponding terminating line rat said second central oiiice which comprises means for adding a `further digit space to each channel exclusive of said message digit spaces, means in each channel `for detecting revertive pulses on the corresponding terminating line at said rst central oiiice, means in each channel to transmit 'binary 0 in said added digit lspace in response to a detected revertive pulse, mea-ns in each channel to transmit binary l in said added digit space in the absence of la detected revertive pulse, and means in each channel for substantially shorting the direct voltage on the corresponding terminating line at said second central oice in response to a received binary 0 in said added digit space.

References Cited in the le of this patent UNITED STATES PATENTS 

2. IN A PULSE CODE MODULATION MESSAGE TRANSMISSION SYSTEM INTERCONNECTING A PAIR OF TERMINALS AND EMPLOYING A PREDETERMINED NUMBER OF DIGIT SPACES FOR MESSAGE TRANSMISSION, AN ARRANGEMENT FOR TRANSMITTING A PAIR OF TWOSTATE DIRECT-CURRENT SIGNALS SIMULTANEOUSLY IN THE SAME DIRECTION BETWEEN SAID TERMINALS WHICH COMPRISES MEANS FOR ADDING A FURTHER DIGIT SPACE EXCLUSIVE OF SAID MESSAGE DIGIT SPACES, MEANS FOR DETECTING THE STATE OF A FIRST OF SAID DIRECT-CURRENT SIGNALS AT A FIRST OF SAID TERMINALS, MEANS TO TRANSMIT BINARY "1" IN SAID ADDED DIGIT SPACE IN RESPONSE TO ONE STATE OF SAID FIRST DIRECT-CURRENT SIGNAL AND BINARY "0" IN RESPONSE TO THE OTHER, MEANS FOR DETECTING THE STATE OF THE SECOND OF SAID DIRECT-CURRENT SIGNALS AT SAID FIRST TERMINALS, MEANS TO TRANSMIT BINARY "1" IN THE LEAST SIGNIFICANT OF SAID MESSAGE DIGIT SPACES IN RESPONSE TO ONE STATE OF SAID SECOND DIRECT-CURRENT SIGNAL AND BINARY "0" IN RESPONSE TO THE OTHER, MEANS TO RECONSTRUCT THE DETECTED STATE OF SAID FIRST DIRECT-CURRENT SIGNAL IN RESPONSE TO THE CON- 